Distortion compensating apparatus, transmitting apparatus, and distortion compensating method

ABSTRACT

An apparatus includes: a unit that stores the look-up table including distortion compensation coefficients; a unit that selects addresses according to an input signal, acquires coefficients stored at the selected addresses, and performs the predistortion of the input signal by using the acquired coefficients; a unit that calculates an error signal by comparing with the input signal a feedback signal that indicates an output of a power amplifier to which a result of the predistortion is inputted; a unit that calculates coefficients from the error signal and the acquired coefficients by using the adaptive algorithm; a unit that, for each of the selected addresses, selects coefficients as adequate coefficients from among the calculated coefficients according to the error signal; and a unit that, for each of the selected addresses, calculates an average value of the adequate coefficients and replaces a stored coefficient in the look-up table with the average value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2010-017983, filed on Jan. 29,2010, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a technique of updatingdistortion compensation coefficients used in predistortion.

BACKGROUND

As wireless communication speeds increase, the bandwidth and the dynamicrange of transmission signals become larger. To minimize the qualitydegradation of the signal, high linearity is required for a transmittingapparatus. Moreover, what is required in terms of the downsizing of theapparatus, reductions in operational costs, environmental issues and thelike is a power amplifier that operates in a highly efficient manner.

However, in a typical power amplifier, the linearity goes against thepower conversion efficiency. When the power amplifier is operated in alinear region that sufficiently backs off from saturated power, theout-of-band distortion can be made smaller. This, however, decreases thepower conversion efficiency significantly, leading to an increase in thepower consumption of the apparatus. A distortion compensating circuit istherefore used to eliminate the nonlinear distortion that emerges whenthe power amplifier is operated in a high-efficiency nonlinear range.

A predistortion method, one of the distortion compensating methods, is atechnique of multiplying the transmission signals by the inverse of thenonlinear distortion of the power amplifier in advance to improve thelinearity of the output from the power amplifier.

A predistortion method that uses digital signal processing is referredto as a digital predistortion (DPD) method. A LUT-based DPD, one of theDPD methods, is widely known: For the LUT-based DPD, distortioncompensation coefficients are kept on a look-up table (LUT) in a memory.What is stored at each LUT address assigned to the LUT is a distortioncompensation coefficient that is based on the amplitude of thetransmission signal. According to the LUT-based DPD, a LUT address isdetermined based on the amplitude of the transmission signal, adistortion compensation coefficient stored at the LUT address is readout, and the distortion compensation coefficient is applied to thetransmission signal to produce a predistortion signal. Moreover,according to the LUT-based DPD, the distortion compensation coefficientis updated based on a signal (feedback signal) that is generated after aportion of the output from the power amplifier is fed back, therebymaking the distortion compensation coefficient vary according to changesin the characteristics of the power amplifier as well as changes overthe years.

What is known as a related technique is a nonlinear distortioncompensation transmitting apparatus, which reduces the convergence timeof distortion compensation coefficients, as well as a distortioncompensating apparatus, which makes corrections in such a way thattransmission signals do not go beyond the dynamic range of a DAconverter.

-   [Patent Document 1] Japanese Laid-open Patent Publication No.    2002-223171-   [Patent Document 2] Japanese Laid-open Patent Publication No.    2001-251148

However, according to the LUT-based DPD, when noise components of afeedback loop are large or when large changes of the characteristicsoccur instantaneously as a low back-off operation of the power amplifieris performed, the accuracy of the updated distortion compensationcoefficients decreases. It may take longer time for the distortioncompensation coefficients to converge into the most appropriate value.

SUMMARY

According to an aspect of the invention, a distortion compensatingapparatus that performs predistortion by using a look-up table andoptimizes the look-up table by using an adaptive algorithm is provided.The apparatus includes: a storage unit that stores the look-up tableincluding distortion compensation coefficients respectively stored ataddresses assigned to the look-up table; a predistortion unit thatselects addresses from among the assigned addresses according to aninput signal, acquires distortion compensation coefficients stored atthe selected addresses, and performs the predistortion of the inputsignal by using the acquired distortion compensation coefficients; anerror calculation unit that calculates an error signal by comparing withthe input signal a feedback signal that indicates an output of a poweramplifier to which a result of the predistortion is inputted; acoefficient calculation unit that calculates distortion compensationcoefficients from the error signal and the acquired distortioncompensation coefficients by using the adaptive algorithm; a coefficientselection unit that, for each of the selected addresses, selectsdistortion compensation coefficients as adequate coefficients from amongthe calculated distortion compensation coefficients according to theerror signal; and a coefficient averaging unit that, for each of theselected addresses, calculates an average value of the adequatecoefficients and replaces a distortion compensation coefficient storedat a corresponding address in the look-up table with the average value.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating the configuration of atransmitting apparatus according to a first embodiment of the presentinvention;

FIG. 2 is a flowchart illustrating a distortion compensation processaccording to the first embodiment;

FIG. 3 is a block diagram illustrating the configuration of acoefficient selection unit and a coefficient averaging unit according tothe first embodiment;

FIG. 4 is a block diagram illustrating the configuration of a DPD unitof a comparative example;

FIG. 5 is a block diagram illustrating the configuration of acoefficient selection unit according to a second embodiment of thepresent invention;

FIG. 6 is a flowchart illustrating first specific address calculationaccording to the second embodiment;

FIG. 7 is a diagram illustrating the amplitude probability distributionof error signals according to the second embodiment;

FIG. 8 is a diagram illustrating the distribution of error signals on acomplex plane according to the second embodiment;

FIG. 9 is a block diagram illustrating the configuration of acoefficient selection unit according to a third embodiment of thepresent invention;

FIG. 10 is a flowchart illustrating first specific address calculationaccording to the third embodiment;

FIG. 11 is a block diagram illustrating the configuration of acoefficient selection unit according to a fourth embodiment of thepresent invention;

FIG. 12 is a block diagram illustrating the configuration of acoefficient selection unit according to a fifth embodiment of thepresent invention;

FIG. 13 is a flowchart illustrating first specific address calculationaccording to the fifth embodiment;

FIG. 14 is a graph illustrating a characteristic of a deviationthreshold value calculation function f(ma_(k)) according to the fifthembodiment;

FIG. 15 is a block diagram illustrating the configuration of acoefficient selection unit according to a sixth embodiment of thepresent invention; and

FIG. 16 is a graph illustrating a characteristic of a deviationthreshold value calculation function g(n) according to the sixthembodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings.

First Embodiment

The following describes a transmitting apparatus 1 a as an example inwhich a transmitting apparatus of the present invention is applied.

FIG. 1 is a block diagram illustrating the configuration of atransmitting apparatus 1 a. The transmitting apparatus 1 a includes aDPD unit 2 a, a DAC (Digital Analog Converter) 3, a modulating unit 4, apower amplifier 5, a demodulating unit 6, and an ADC (Analog DigitalConverter) 7. The DPD unit 2 a is an example of the application of adistortion compensating apparatus of the present invention.

A transmission signal, a digital signal transmitted by the transmittingapparatus 1 a, is inputted into the DPD unit 2 a. For example, thetransmission signal is a sequence of samples that is generated after thewaveform of a baseband signal or intermediate-frequency (IF) signal issampled at predetermined sampling intervals; the transmission signal isa sequence of complex samples having a real part (I component) and animaginary part (Q component).

The DPD unit 2 a performs predistortion on the transmission signal tocompensate for distortion and outputs the compensated transmissionsignal, the transmission signal on which predistortion has beenperformed.

The DAC 3 converts the compensated transmission signal from the DPD unit2 a to an analog signal. The modulating unit 4 includes a carriergenerator 11 a and a modulator 12 a. The carrier generator 11 agenerates a sine wave having a predetermined transmission carrierfrequency. The modulator 12 a upconverts the transmission signal bymultiplying the output from the DAC 3 with the output from the carriergenerator 11 a.

The power amplifier 5 amplifies the output from the modulating unit 4 togenerate a PA output signal. The output signal contains nonlineardistortion given by the power amplifier 5. When the transmittingapparatus 1 performs wireless transmission, the output from the poweramplifier 5 is sent to an antenna. When the transmitting apparatus 1 aperforms wired transmission, the output from the power amplifier 5 isoutputted to a transmission channel.

The demodulating unit 6 includes a carrier generator 11 b and amodulator 12 b. The carrier generator 11 b generates a sine wave havinga predetermined reception carrier frequency. The modulator 12 bdownconverts the feedback signal by multiplying a portion of the outputfrom the power amplifier 5 with the output from the carrier generator 11b. The ADC 7 converts the feedback signal outputted from thedemodulating unit 6 to a digital signal.

The following describes the DPD unit 2 a.

The DPD unit 2 a includes a REF (Reference) signal buffer 21, a FB(Feedback) signal buffer 22, a subtracter 23, an address generation unit31, an address buffer 32, a coefficient averaging unit 33, a LUT 34, acoefficient buffer 35, a multiplier 36, a coefficient selection unit 37a, and a coefficient calculation unit 38. The coefficient calculationunit 38 includes a complex conjugate arithmetic unit 24, a multiplier25, a multiplier 26, and an adder 27.

The length of the address buffer 32, REF signal buffer 21, FB signalbuffer 22, and coefficient buffer 35 is buffer length N. In this case, Nis a positive integer. The address buffer 32 stores N samples of LUTaddresses from the address generation unit 31. The REF signal buffer 21stores N samples of transmission signals inputted to the transmittingapparatus 1 a. The FB signal buffer 22 stores N samples of feedbacksignals outputted from the ADC 7. The coefficient buffer 35 stores Nsamples of distortion compensation coefficients outputted from the LUT34. Hereinafter, a period of N samples is defined as one buffer period.

The LUT 34 stores M distortion compensation coefficients at M LUTaddresses. The distortion compensation coefficient stored at a given LUTaddress is multiplied by the transmission signal corresponding to theLUT address. Here, M represents the number of LUT addresses and is apositive integer. The LUT address is defined as k. In this case, k is aninteger ranging from 0 to M−1.

The following describes a coefficient calculation method by which theDPD unit 2 a calculates a new distortion compensation coefficient from adistortion compensation coefficient in the LUT 34.

The coefficient calculation method uses an adaptive algorithm. Here, forease of explanation, a loop delay, a processing delay and the like arenot taken into account. With one buffer period as a unit, at time n, thetransmission signal for one buffer period in the REF signal buffer 21 isdefined as x[n], the feedback signal for one buffer period in the FBsignal buffer 22 as y[n], and the distortion compensation coefficientfor one buffer period in the coefficient buffer 35 as h[n]. An errorsignal that represents the difference between the transmission signalx[n], which is a reference signal, and the feedback signal is defined ase[n]. With the use of the reference signal stored in the REF signalbuffer 21, the feedback signal stored in the FB signal buffer 22, andthe distortion compensation coefficient, the DPD unit 2 a calculates anupdated coefficient h[n+1], a new distortion compensation coefficient,using the following equation 1.h[n+1]=h[n]+μ×e[n]×h[n]×y[n]*  (Equation 1)

Here, μ represents a step-size parameter, and y[n]* the complexconjugate of y[n]. By updating the distortion compensation coefficientas illustrated in Equation 1, the updated coefficient h[n+1] for onebuffer period is generated.

The LUT address for one buffer period in the address buffer 32 isdefined as k[n]. Each k[n] is one of the integers 0 to M−1. A samplenumber that is referred to in the address buffer 32, the REF signalbuffer 21 and the FB signal buffer 22 is defined as i. Here, i is aninteger ranging from 0 to N−1.

Among the LUT addresses k[n] of N samples in the address buffer 32, theLUT address that is read out in accordance with the sample number i isdefined as k_i, with i serving as an index. Similarly, among thetransmission signals x[n] of N samples in the REF signal buffer 21, thetransmission signal that is read out in accordance with the samplenumber i is defined as x_i. Among the feedback signals y[n] of N samplesin the FB signal buffer 22, the feedback signal that is read out inaccordance with the sample number i is defined as y_i. Among thedistortion compensation coefficients h[n] of N samples in thecoefficient buffer 35, the distortion compensation coefficient that isread out in accordance with the sample number i is defined as h_i.Similarly, among the updated coefficients h[n+1], the updatedcoefficient that is calculated in accordance with the sample number i isdefined as hu_i. Among the error signals e[n], the error signal that iscalculated in accordance with the sample number i is defined as e_i.

The transmission signal x_i, the LUT address k_i, the feedback signaly_i, and the distortion compensation coefficient h_i are read out fromthe REF signal buffer 21, the FB signal buffer 22, the address buffer 32and the coefficient buffer 35, respectively, in such a way that thetransmission signal x_i, the LUT address k_i, the feedback signal y_i,and the distortion compensation coefficient h_i synchronize with eachother in accordance with the sample number i. The LUT address that isdetermined by the address generation unit 31 from the transmissionsignal x_i is k_i. The distortion compensation coefficient by which themultiplier 36 multiplies the transmission signal x_i is h_i. Thefeedback signal that represents the result that the power amplifier 5obtains by amplifying the result of multiplying the transmission signalx_i and the distortion compensation coefficient h_i together is y_i.

The following describes a distortion compensation process that the DPDunit 2 a performs.

FIG. 2 is a flowchart illustrating a distortion compensation process.The distortion compensation process is performed for each buffer period.

The address generation unit 31 measures a predetermined parameter of thetransmission signal x_i and selects the LUT address k_i that representsthe measured predetermined parameter (S211). In the example here, thepredetermined parameter is the amplitude (absolute value) of thetransmission signal x_i. In this case, the address generation unit 31has stored in advance the relationships between a plurality of ranges ofthe amplitude and a plurality of LUT addresses. The multiplier 36 readsout the distortion compensation coefficient h_i stored at the LUTaddress k_i of the LUT 34 (S212), multiplies the distortion compensationcoefficient h_i and the transmission signal x_i together to performpredistortion on the transmission signal x_i, generates the compensatedtransmission signal, which is the result of the predistortion, andoutputs the compensated transmission signal to the DAC 3 (S213).

The subtracter 23 subtracts the feedback signal y_i in the FB signalbuffer 22 from the transmission signal x_i in the REF signal buffer 21to calculate the error signal e_i (S221). The coefficient calculationunit 38 uses the above coefficient calculation method to calculate theupdated coefficient hu_i from the distortion compensation coefficienth_i, the error signal e_i and the feedback signal y_i (S222). Thecoefficient selection unit 37 a selects, from among the updatedcoefficients hu_i, an adequate coefficient hs_i according to the errorsignal e_i (S231). The coefficient averaging unit 33 calculates anaverage coefficient ha_(k), which is the average of the adequatecoefficients hs_i that are calculated in response to the LUT addresses k(S242). The coefficient averaging unit 33 overwrites (replaces) thedistortion compensation coefficient stored at the LUT address k in theLUT 34 with the average coefficient ha_(k) (S243).

The distortion compensation process ends with the above process. Thedistortion compensation process is then repeated for each buffer period.Among the processes of the distortion compensation process, theprocesses S221 to S243 are defined as a coefficient updating process inwhich the distortion compensation coefficients in the LUT 34 areupdated.

The predetermined parameter measured by the address generation unit 31may represent the phase of the transmission signal x_i, or the amplitudeand phase of the transmission signal x_i.

The following describes the coefficient calculation unit 38.

The complex conjugate arithmetic unit 24 calculates the complexconjugate signal y_i* of the feedback signal y_i. The multiplier 25multiplies h_i in the coefficient buffer 35 and the complex conjugatesignal y_i* together. The multiplier 26 multiplies the error signal e_icalculated by the subtracter 23, a predetermined step-size parameter μ,and the output from the multiplier 25 together. The adder 27 adds h_i inthe coefficient buffer 35 and the output from the multiplier 26 tocalculate the updated coefficient hu_i. The error signal e_i, the LUTaddress k_i that is stored in the address buffer 32, and the updatedcoefficient hu_i are inputted into the coefficient selection unit 37 a.

The following describes the coefficient selection unit 37 a and thecoefficient averaging unit 33.

FIG. 3 is a block diagram illustrating the configuration of thecoefficient selection unit 37 a and the coefficient averaging unit 33.The coefficient selection unit 37 a includes one-input, M-outputselectors (demultiplexers) 51 and 52 and M first specific addressarithmetic units 53 a. The coefficient averaging unit 33 includes Msecond specific address arithmetic units 61 and a M-input, one-outputselector (multiplexer) 62.

The following describes the coefficient selection unit 37 a.

As the sample number i increases, the LUT addresses k_i are sequentiallyinputted from the address buffer 32 to control terminals of theselectors 51 and 52. Similarly, the error signals e_i are sequentiallyinputted to an input terminal of the selector 51. Similarly, the updatedcoefficients hu_i are sequentially inputted to an input terminal of theselector 52.

The selector 51 includes one control terminal, one input terminal, and Moutput terminals eo₀, eo₁, . . . , eo_(k), . . . , and eo_(M-1)corresponding to the M LUT addresses. In the selector 51, when the LUTaddress k is inputted to the control terminal, the output terminaleo_(k) corresponding to the LUT address k is selected, and the signalinputted to the input terminal is outputted to the selected outputterminal eo_(k). Similarly, the selector 52 includes one controlterminal, one input terminal, and M output terminals huo₀, huo₁, . . . ,huo_(k), . . . , and huo_(M-1) corresponding to the M LUT addresses. Inthe selector 52, when the LUT address k is inputted to the controlterminal, the output terminal huo_(k) corresponding to the LUT address kis selected, and the signal inputted to the input terminal is outputtedto the selected output terminal huo_(k).

The M first specific address arithmetic units 53 a are so provided as tocorrespond to the M LUT addresses. The M first specific addressarithmetic units 53 a each include a condition definition unit 71 a, adetermination unit 72, and a two-input, one-output selector(multiplexer) 73.

The following describes first specific address calculation for the LUTaddress k.

When the LUT address k_i that is inputted into the coefficient selectionunit 37 a is k, the error signal e_i inputted to the input terminal ofthe selector 51 is outputted from the output terminal eo_(k), which isso provided as to correspond to k. The error signal e_i from the outputterminal eo_(k) is inputted into the first specific address arithmeticunit 53 a corresponding to the LUT address k. Similarly, when the LUTaddress k_i that is inputted into the coefficient selection unit 37 a isk, the updated coefficient hu_i inputted to the input terminal of theselector 52 is outputted from the output terminal huo_(k), which is soprovided as to correspond to k. The updated coefficient hu_i from outputterminal huo_(k) of the selector 52 is inputted into the first specificaddress arithmetic unit 53 a corresponding to the LUT address k.

A fixed value 0 is inputted to a first input, which is one of the twoinputs of the selector 73, and the updated coefficient hu_i from theoutput terminal huo_(k) of the selector 52 to the second input. Thecondition definition unit 71 a defines a selection condition accordingto the error signal e_i that is inputted within one buffer period. Thedetermination unit 72 makes a determination as to whether the errorsignal e_i meets the selection condition each time the error signal e_iand the updated coefficient hu_i are inputted. When the error signal e_imeets the selection condition, the determination unit 72 makes theselector 73 select the second input. When the error signal e_i does notmeet the selection condition, the determination unit 72 makes theselector 73 select the first input. Therefore, when the error signal e_imeets the selection condition, the selector 73 outputs the updatedcoefficient hu_i to an output terminal hso_(k). When the error signale_i does not meet the selection condition, the selector 73 does notoutput the updated coefficient hu_i to an output terminal hso_(k).

The following describes the coefficient averaging unit 33.

The M second specific address arithmetic units 61 are so provided as tocorrespond to the M LUT addresses.

Here, second specific address calculation for the LUT address k will bedescribed. Among the updated coefficients hu_i, the one output from thecoefficient selection unit 37 a to the coefficient averaging unit 33 isdefined as the adequate coefficient hs_i. When the adequate coefficienths_i is outputted from the output terminal hso_(k) of the first specificaddress arithmetic unit 53 a corresponding to the LUT address k, theadequate coefficient hs_i is inputted into the second specific addressarithmetic unit 61 corresponding to the LUT address k. The secondspecific address arithmetic unit 61 corresponding to the LUT address kaverages the adequate coefficients hs_i that are inputted within onebuffer period to calculate the average coefficient ha_(k). All theaverage coefficients ha_(k) that are outputted from the coefficientaveraging unit 33 with k ranging from 0 to M−1 are represented by ha.

After the averaging of one buffer period is completed by the secondspecific address arithmetic unit 61, the LUT addresses k for 0 to M−1are sequentially inputted to the control terminal of the selector 62 andwriting addresses of the LUT 34. The selector 62 includes one controlterminal, M input terminals hai₀, hai₁, . . . , hai_(k), . . . , andhai_(M-1), and one output terminal. In the selector 62, when the LUTaddress k is inputted to the control terminal, the signal inputted tothe input terminal corresponding to the LUT address k is selected andoutputted to the output terminal.

Then, the average coefficient ha_(k) calculated by the second specificaddress arithmetic unit 61 corresponding to the LUT address k isinputted to the input terminal hai_(k) of the selector 62 thatcorresponds to the LUT address k. In accordance with the LUT address kinputted to the control terminal, the selector 62 selects the inputterminal hai_(k) and outputs to the output terminal the averagecoefficient ha_(k) inputted to the selected input terminal hai_(k). Insynchronization with the above, the LUT address k is inputted to awriting address of the LUT 34. Therefore, the average coefficientsha_(k) that correspond to the LUT addresses k for 0 to M−1 aresequentially written on the LUT addresses k of the LUT 34 so that thedistortion compensation coefficients of the LUT 34 are updated. Onecoefficient updating process ends with the above process.

As described above, the coefficient selection unit 37 a outputs to thecoefficient averaging unit 33 only the adequate coefficient hs_i, whichis the updated coefficient hu_i whose error signal e_i meets theselection condition among the updated coefficients hu_i. The coefficientaveraging unit 33 averages the adequate coefficients hs_i within onebuffer period for each LUT address k.

Incidentally, the DPD unit 2 a may be realized by a circuit or the like,or by a combination of a processor and a memory such as a DSP (DigitalSignal Processor) or computer. When the DPD unit 2 a is realized by acombination of a processor and a memory, the memory stores software thatenables the processor to perform the function of the DPD unit 2 a, andthe processor performs the function of the DPD unit 2 a in accordancewith the software.

The following describes a DPD unit 2 x, which is a comparative exampleof the DPD unit 2 a.

FIG. 4 is a block diagram illustrating the configuration of the DPD unit2 x. As for the DPD unit 2 x, the same reference symbols as those of theDPD unit 2 a represent components that are the same as, or similar to,those of the DPD unit 2 a; the components therefore will not bedescribed here. Compared with the DPD unit 2 a, the DPD unit 2 x doesnot include the coefficient selection unit 37 a. All the averagecoefficients ha_(k) that are outputted from the coefficient averagingunit 33 with k ranging from 0 to M−1 are represented by hax.

That is, the coefficient averaging unit 33 of the DPD unit 2 x of thecomparative example averages all the updated coefficients hu_i for eachLUT address to calculate the average coefficients hax. Accordingly, dueto the effects of the updated coefficients generated from the feedbacksignals containing noise and instantaneous changes in thecharacteristics of the output of the power amplifier, the accuracy ofthe average coefficients hax decreases, and it may take longer time forthe distortion compensation coefficients to converge into the mostappropriate value.

Meanwhile, the DPD unit 2 a of the first embodiment includes thecoefficient selection unit 37 a that is followed by the coefficientaveraging unit 33. Accordingly, the coefficient averaging unit 33averages only the adequate coefficients hs_i for each LUT address tocalculate the average coefficients ha. That is, in averaging theadequate coefficients hs_i, the coefficient averaging unit 33 of the DPDunit 2 a does not use the updated coefficients generated from thefeedback signals containing noise and instantaneous changes in thecharacteristics of the power amplifier 5. Therefore, the accuracy of theaverage coefficients ha improves, and it is possible to reduce the timeneeded for the distortion compensation coefficients to converge into themost appropriate value. Thus, even under the operational condition thatchanges in the characteristics of the output of the power amplifier 5occur, it is possible to successfully lower the out-of-band distortion.

Hereinafter, another embodiment of the distortion compensating apparatusof the present embodiment will be described. The distortion compensatingapparatus (which is for example the DPD unit 2 a) updates, atpredetermined intervals, a distortion compensation coefficient by whichmultiplied is an input signal value (which is for example a transmissionsignal) that is sequentially inputted to the distortion compensatingapparatus. The distortion compensating apparatus also includes a storageunit (which is for example the LUT 34) that stores in a storage uniteach of a plurality of the distortion compensation coefficients at eachof a plurality of addresses that represent values of predeterminedparameters (which for example represent amplitude) of the input signalvalues. Moreover, the distortion compensating apparatus includes apredistortion unit (which is for example the multiplier 36) that selectsan application address, which is an address representing the value ofthe predetermined parameter of the input signal value, from among aplurality of the addresses for each input signal value and multiplies anapplication coefficient, which is a distortion compensation coefficientstored at the application address in the storage unit, and the inputsignal value together. Furthermore, the distortion compensatingapparatus includes an error calculation unit (which is for example thesubtracter 23) that acquires, after a signal that is based on the resultof the multiplication is amplified by an amplifier (which is for examplethe power amplifier 5), an amplified signal value that is based on theresult of the amplification for each input signal value and calculatesthe error in the amplified signal value relative to the input signalvalue. Furthermore, the distortion compensating apparatus includes acoefficient calculation unit (which is for example the coefficientcalculation unit 38) that calculates an estimated coefficient as a newdistortion compensation coefficient according to the applicationcoefficient, the error and the amplified signal value. Furthermore, thedistortion compensating apparatus includes a coefficient selection unit(which is for example the coefficient selection unit 37 a) that acquiresa plurality of errors, which are calculated from a plurality of theinput signal values through the calculation of the errors, and aplurality of estimated coefficients, which are calculated from aplurality of the input signal values through the calculation of theestimated coefficients, and selects a plurality of adequatecoefficients, which are a plurality of distortion compensationcoefficients among a plurality of the estimated coefficients, accordingto a plurality of the errors. Furthermore, the distortion compensatingapparatus includes a coefficient averaging unit (which is for examplethe coefficient averaging unit 33) that selects each of a plurality oftarget coefficients, which are adequate coefficients calculated from thedistortion compensation coefficients stored at target addresses in thestorage unit, from among a plurality of the adequate coefficients foreach of the target addresses among a plurality of addresses, calculatesan average coefficient, which is the average of a plurality of thetarget coefficients, and writes the average coefficient at the targetaddress in the storage unit.

Second Embodiment

Hereinafter, another embodiment of the coefficient selection unit 37 awill be described.

FIG. 5 is a block diagram illustrating the configuration of thecoefficient selection unit 37 a of a second embodiment of the presentinvention. In the diagram, the same reference symbols as those of theelements in the coefficient selection unit 37 a of the first embodimentrepresent components that are the same as, or similar to, thoseillustrated in the first embodiment; the components therefore will notbe described here. Compared with the coefficient selection unit 37 a ofthe first embodiment, the coefficient selection unit 37 a of the secondembodiment includes a plurality of first specific address arithmeticunits 53 b instead of a plurality of the first specific addressarithmetic units 53 a. Compared with the first specific addressarithmetic units 53 a, the first specific address arithmetic units 53 beach include a condition definition unit 71 b instead of a conditiondefinition unit 71 a.

The following describes first specific address calculation performed bythe first specific address arithmetic units 53 b.

FIG. 6 is a flowchart illustrating the first specific addresscalculation according to the second embodiment. What is described hereis the first specific address calculation performed by the firstspecific address arithmetic unit 53 b corresponding to the LUT addressk.

First, the condition definition unit 71 b calculates instantaneous erroramplitude m_i, which is the amplitude (absolute value) of the errorsignal e_i from the output terminal eo_(k) of the selector 51, for eachsample number i (S11). Then, the condition definition unit 71 b averagesthe instantaneous error amplitudes m_i during one buffer period tocalculate average error amplitude ma_(k) (S12).

Subsequently, the condition definition unit 71 b calculates a standarddeviation σ_(k) of the instantaneous error amplitudes m_i according tothe instantaneous error amplitudes m_i and the average error amplitudema_(k) (S21). The condition definition unit 71 b then defines aselection condition according to the standard deviation σ_(k) (S22). Theselection condition here is that deviation d_i is smaller than adeviation threshold value W_(k). The deviation d_i is the absolute valueof the difference between the instantaneous error amplitude m_i and theaverage error amplitude ma_(k). The deviation threshold value W_(k) isdefined as K×σ_(k). In this case, K is a predetermined positive number.That is, the condition definition unit 71 b determines the deviationthreshold value W_(k) according to the standard deviation σ_(k).

Subsequently, the determination unit 72 initializes the sample number iat 0 (S110).

The determination unit 72 then makes a determination as to whether thesample number i is greater than or equal to buffer length N (S111). Thatis, the determination unit 72 makes a determination as to whether theprocess for all samples within one buffer period has been completed.

When the sample number i is greater than or equal to N (S111, Yes), theflow ends.

When the sample number i is less than N (S111, No), the determinationunit 72 makes a determination as to whether the error signal e_i isinputted from the output terminal eo_(k) of the selector 51 to the firstspecific address arithmetic units 53 b (S112).

When the error signal e_i is not inputted (S112, No), the flow proceedsto S130.

When the error signal e_i is inputted (S112, Yes), the determinationunit 72 calculates the deviation d_i, the absolute value of thedifference between the instantaneous error amplitude m_i and the averageerror amplitude ma_(k) (S121). Then, the determination unit 72 makes adetermination as to whether the deviation d_i is smaller than thedeviation threshold value W_(k) (S122).

When the deviation d_i is smaller than the deviation threshold valueW_(k) (S122, Yes), the determination unit 72 outputs the updatedcoefficient hu_i from the adder 27 to the coefficient averaging unit 33as the adequate coefficient hs_i (S123), and the flow proceeds to S130.

When the deviation d_i is not smaller than the deviation threshold valueW_(k) (S122, No), the determination unit 72 does not output the updatedcoefficient hu_i from the adder 27 to the coefficient averaging unit 33,and the flow proceeds to S130. When the error signal e_i is not inputtedat S112 (S112, No), or when the determination unit 72 outputs theadequate coefficient hs_i to the coefficient averaging unit 33 at stepS123, or when the deviation d_i is not smaller than the deviationthreshold value W_(k) at step S122 (S122, No), the determination unit 72increases the sample number i by 1 (S130) and the flow proceeds to S111where the process for the next sample number takes place.

As described above, the coefficient selection unit 37 a transfers theupdated coefficient hu_i only when the deviation d_i is smaller than thedeviation threshold value W_(k) to the coefficient averaging unit 33 asthe adequate coefficient hs_i. Therefore, the coefficient averaging unit33 can average the updated coefficients and update the distortioncompensation coefficients using only the adequate coefficient hs_ioutputted from the coefficient selection unit 37 a.

The following describes the selection condition.

FIG. 7 is a diagram illustrating the amplitude probability distributionof error signals. In the diagram, the horizontal axis represents theinstantaneous error amplitude m_i; the vertical axis representsprobability (sample number). In the diagram, ma_(k) on the horizontalaxis represents the average error amplitude; the shaded area representsa region that satisfies the selection condition. The region thatsatisfies the selection condition is defined by the deviation thresholdvalue W_(k). The region that satisfies the selection condition is aregion where the instantaneous error amplitude m_i is greater than (theaverage error amplitude ma_(k)−the deviation threshold value W_(k)) andless than (the average error amplitude ma_(k)+the deviation thresholdvalue W_(k)).

As described above, the transmission signal x_i is a complex signal.Therefore, the error signal e_i is a complex signal. FIG. 8 is a diagramillustrating the distribution of error signals on a complex plane. Inthe diagram, the horizontal axis represents the real part (I component)of the error signals e_i; the vertical axis represents the imaginarypart (Q component) of the error signals e_i. Also in the diagram, aplurality of points indicated by “x” represents the error signals e_iwithin one buffer period. Moreover, in the diagram, a circle with aradius of W_(k) represents the selection condition. The area inside thecircle represents a region of the error signals e_i where the selectioncondition is satisfied. The center of the circle represents the averageof the error signals e_i.

According to the selection conditions illustrated in the presentembodiment, the coefficient selection unit 37 a determines the selectioncondition according to the variation of the error signal e_i andtransfers only the updated coefficient hu_i generated from the errorsignal e_i that meets the selection condition to the coefficientaveraging unit 33 as the adequate coefficient hs_i.

Incidentally, the selection condition may be defined by a range of theerror signals e_i, a range of the instantaneous error amplitude m_i, orthe like.

Incidentally, the coefficient selection unit 37 a may determine oneselection condition from all the error signals e_i within one bufferperiod regardless of the LUT addresses, instead of determining theselection condition for each LUT address. In this case, the coefficientselection unit 37 a for example determines one deviation threshold valuefrom all the error signals e_i within one buffer period regardless ofthe LUT addresses.

Third Embodiment

Hereinafter, another embodiment of the coefficient selection unit 37 awill be described.

FIG. 9 is a block diagram illustrating the configuration of thecoefficient selection unit 37 a of a third embodiment of the presentinvention. In the diagram, the same reference symbols as those of theelements in the coefficient selection unit 37 a of the second embodimentrepresent components that are the same as, or similar to, thoseillustrated in the second embodiment; the components therefore will notbe described here. Compared with the coefficient selection unit 37 a ofthe second embodiment, the coefficient selection unit 37 a of the thirdembodiment includes a plurality of first specific address arithmeticunits 53 c instead of a plurality of the first specific addressarithmetic units 53 b, as well as a new selection condition storage unit74 c. Compared with the first specific address arithmetic units 53 b,the first specific address arithmetic units 53 c each include acondition definition unit 71 c instead of a condition definition unit 71b.

The following describes first specific address calculation performed bythe first specific address arithmetic units 53 c.

The selection condition storage unit 74 c is, for example, a memory thatstores a selection condition table. The selection condition tablecontains information about M selection conditions, which are defined inadvance so as to correspond to M LUT addresses. In the example here, theselection condition for the LUT address k is that the deviation d_i issmaller than a deviation threshold value Wt_(k). In the example here,the selection condition table contains M deviation threshold valuesWt_(k), which are defined in advance so as to correspond to the M LUTaddresses.

FIG. 10 is a flowchart illustrating the first specific addresscalculation according to the third embodiment. What is described here isthe first specific address calculation performed by the first specificaddress arithmetic unit 53 c corresponding to the LUT address k. First,the condition definition unit 71 c performs the same processes S11 andS12 as in the second embodiment. Then, the condition definition unit 71c reads out from the selection condition table in the selectioncondition storage unit 74 c Wt_(k) that is so stored as to correspond tothe LUT address k_i; and then determines that the Wt_(k) that thecondition definition unit 71 c has read out is the deviation thresholdvalue W_(k) (S31). The determination unit 72 then carries out the sameprocesses S110 to S130 as in the second embodiment.

According to the selection conditions illustrated in the presentembodiment, the first specific address calculation uses the selectionconditions that are set in advance for the LUT addresses, making theprocesses of the condition definition unit 71 c simple.

Fourth Embodiment

Hereinafter, another embodiment of the coefficient selection unit 37 awill be described.

FIG. 11 is a block diagram illustrating the configuration of thecoefficient selection unit 37 a according to a fourth embodiment of thepresent invention. In the diagram, the same reference symbols as thoseof the elements in the coefficient selection unit 37 a of the thirdembodiment represent components that are the same as, or similar to,those illustrated in the third embodiment; the components therefore willnot be described here. Compared with the coefficient selection unit 37 aof the third embodiment, the coefficient selection unit 37 a of thefourth embodiment includes a plurality of first specific addressarithmetic units 53 d instead of a plurality of the first specificaddress arithmetic units 53 c; a selection condition storage unit 74 dinstead of the selection condition storage unit 74 c; and a newcharacteristic value calculation unit 75 d. Compared with the firstspecific address arithmetic units 53 c, the first specific addressarithmetic units 53 d each include a condition definition unit 71 dinstead of the condition definition unit 71 c.

The characteristic value calculation unit 75 d reads out a transmissionsignal stored in the REF signal buffer 21 and calculates acharacteristic value of the transmission signal. The characteristicvalue is, for example, the average power of the transmission signal, thebandwidth of the transmission signal, the PAPR (peak-to-average powerratio) of the transmission signal, or the like. For ranges of thecharacteristic value, a plurality of different characteristic-valueranges is defined. The characteristic value calculation unit 75 d thendetermines a characteristic-value range that contains the characteristicvalue calculated by the characteristic value calculation unit 75 d.

The selection condition storage unit 74 d stores a plurality ofselection condition tables, which are defined in advance so as tocorrespond to a plurality of the above characteristic-value ranges. Forexample, a plurality of the selection condition tables each includes, asin the case of the selection condition tables of the third embodiment, Mdeviation threshold values corresponding to M LUT addresses.

Compared with the first specific address arithmetic units 53 c, theprocess at S31 of determining the deviation threshold value W_(k) isdifferent for the first specific address calculation by the firstspecific address arithmetic units 53 d. At S31, the condition definitionunit 71 d of the first specific address arithmetic units 53 d selectsone selection condition table that is so defined as to correspond to acharacteristic-value range determined by the characteristic valuecalculation unit 75 d from among a plurality of the selection conditiontables in the selection condition storage unit 74 d. Then, the conditiondefinition unit 71 d defines, as in the case of the condition definitionunit 71 c, a selection condition according to the selected selectioncondition table.

If the transmission signal is wideband, a change in the characteristicsof the output of the power amplifier 5 typically increases due to thememory effect of the amplifier. In this case, the condition definitionunit 71 d for example uses a set of large deviation threshold valuesWt_(k) across the entire range of the LUT address k. If the poweramplifier 5 is operated by the average power that backs off fromsaturated power, nonlinear distortion decreases. In this case, what isused by the condition definition unit 71 d is, for example, a selectioncondition table where the deviation threshold values W_(k) are generallysmall, a selection condition table with small deviation threshold valuesW_(k) that is defined so as to correspond to the LUT address kindicating the case in which the amplitude of the transmission signal issmall, or the like.

According to the selection conditions illustrated in the presentembodiment, the coefficient selection unit 37 a selects a suitableselection condition table for the characteristic value of thetransmission signal from among a plurality of the selection conditiontables. Therefore, it is possible to vary the selection conditionaccording to the characteristics of the transmission signal.

Fifth Embodiment

Hereinafter, another embodiment of the coefficient selection unit 37 awill be described.

FIG. 12 is a block diagram illustrating the configuration of thecoefficient selection unit 37 a according to a fifth embodiment of thepresent invention. In the diagram, the same reference symbols as thoseof the elements in the coefficient selection unit 37 a of the secondembodiment represent components that are the same as, or similar to,those illustrated in the second embodiment; the components thereforewill not be described here. Compared with the coefficient selection unit37 a of the second embodiment, the coefficient selection unit 37 a ofthe fifth embodiment includes a plurality of first specific addressarithmetic units 53 e instead of a plurality of the first specificaddress arithmetic units 53 b. Compared with the first specific addressarithmetic units 53 b, the first specific address arithmetic units 53 eeach include a condition definition unit 71 e instead of the conditiondefinition unit 71 b. The condition definition unit 71 e has a deviationthreshold value calculation function W_(k)=f(ma_(k)) for determining thedeviation threshold value W_(k) from the average error amplitude ma_(k).

The following describes first specific address calculation by the firstspecific address arithmetic units 53 e.

FIG. 13 is a flowchart illustrating the first specific addresscalculation according to the fifth embodiment. What is described here isthe first specific address calculation performed by the first specificaddress arithmetic unit 53 e corresponding to the LUT address k. First,the condition definition unit 71 e performs the same processes S11 andS12 as in the second embodiment. Then, the condition definition unit 71e calculates the deviation threshold value W_(k) from the calculatedaverage error amplitude ma_(k) and the deviation threshold valuecalculation function f(ma_(k)) (S51). Subsequently, the determinationunit 72 carries out the same processes S110 to S130 as in the secondembodiment.

FIG. 14 is a graph illustrating a characteristic of the deviationthreshold value calculation function f(ma_(k)). In the diagram, thehorizontal axis represents the average error amplitude ma_(k); thevertical axis represents the deviation threshold value W_(k). In theexample here for the deviation threshold value calculation functionf(ma_(k)), as the average error amplitude ma_(k) decreases, thedeviation threshold value W_(k) becomes smaller and the slope of thedeviation threshold value W_(k) relative to the average error amplitudema_(k) increases.

The distortion compensation coefficients further converge as thecoefficient updating process is repeated. Therefore, the feedback signalbecomes similar to the reference signal (transmission signal). Thus, forthe error signal e[n], the average error amplitude ma_(k) and thestandard deviation σ_(k) becomes smaller. Accordingly, with the use ofthe deviation threshold value calculation function f(ma_(k)) having theabove characteristic, the deviation threshold value W_(k) becomessmaller as the distortion compensation coefficients converge further;the accuracy of the updated coefficient, which is used in the averagingprocess, improves, making it possible to reduce the convergence time.

Sixth Embodiment

Hereinafter, another embodiment of the coefficient selection unit 37 awill be described.

FIG. 15 is a block diagram illustrating the configuration of thecoefficient selection unit 37 a according to a sixth embodiment of thepresent invention. In the diagram, the same reference symbols as thoseof the elements in the coefficient selection unit 37 a of the secondembodiment represent components that are the same as, or similar to,those illustrated in the second embodiment; the components thereforewill not be described here. Compared with the coefficient selection unit37 a of the second embodiment, the coefficient selection unit 37 a ofthe sixth embodiment includes a plurality of first specific addressarithmetic units 53 f instead of a plurality of the first specificaddress arithmetic units 53 b, as well as a new update number counter 76f. Compared with the first specific address arithmetic units 53 b, thefirst specific address arithmetic units 53 f each include a conditiondefinition unit 71 f instead of the condition definition unit 71 b.

The update number counter 76 f counts an update number n, which is thenumber of times the coefficient updating process is performed since apredetermined point in time. The condition definition unit 71 f containsa deviation threshold value calculation function W_(k)=g(n) fordetermining the deviation threshold value W_(k) from the update numbern. Incidentally, the condition definition unit 71 f may use, instead ofthe update number n, a parameter indicating how much time has passedsince the predetermined point in time in determining the deviationthreshold value W_(k).

Compared with the above first specific address arithmetic units 53 e,the process at S51 of calculating the deviation threshold value W_(k) isdifferent for the first specific address calculation by the firstspecific address arithmetic units 53 f. At S51, the condition definitionunit 71 f of the first specific address arithmetic unit 53 f calculatesthe deviation threshold value W_(k) from the update number n, which iscounted by the update number counter 76 f, and the deviation thresholdvalue calculation function g(n).

FIG. 16 is a graph illustrating a characteristic of the deviationthreshold value calculation function g(n). In the diagram, thehorizontal axis represents the update number n; the vertical axisrepresents the deviation threshold value W_(k). In the example here forthe deviation threshold value calculation function g(n), as the updatenumber n increases, the deviation threshold value W_(k) becomes smallerand the slope of the deviation threshold value W_(k) relative to theupdate number n decreases.

The distortion compensation coefficients further converge as thecoefficient updating process is repeated. Therefore, the feedback signalbecomes similar to the reference signal. Thus, the average erroramplitude ma_(k) and the standard deviation σ_(k) becomes smaller.Accordingly, with the use of the deviation threshold value calculationfunction g(n) having the above characteristic, the deviation thresholdvalue W_(k) becomes smaller as the distortion compensation coefficientsconverge further; the accuracy of the updated coefficient, which is usedin the averaging process, improves, making it possible to reduce theconvergence time.

According to the technique disclosed in the present application, it ispossible to reduce the convergence time of the distortion compensationcoefficients.

A storage unit includes the LUT 34. A power amplifier includes the poweramplifier 5. Predistortion includes the address generation unit 31 andthe multiplier 36. An error calculation unit includes the subtracter 23.A coefficient calculation unit includes the coefficient calculation unit38. A coefficient selection unit includes the coefficient selection unit37 a. A coefficient averaging unit includes the coefficient averagingunit 33. Addresses include a LUT address. Input signals include atransmission signal. Errors include an error signal.

The reference numeral 1 a represents a transmitting apparatus. Thereference numeral 2 a represents a DPD unit. The reference numeral 3represents a DAC. The reference numeral 4 represents a modulating unit.The reference numeral 5 represents a power amplifier. The referencenumeral 6 represents a demodulating unit. The reference numeral 7represents an ADC. The reference numerals 11 a and 11 b representcarrier generators. The reference numerals 12 a and 12 b representmodulators. The reference numeral 21 represents a REF signal buffer. Thereference numeral 22 represents a FB signal buffer. The referencenumeral 23 represents a subtracter. The reference numeral 24 representsa complex conjugate arithmetic unit. The reference numeral 25 representsa multiplier. The reference numeral 26 represents a multiplier. Thereference numeral 27 represents an adder. The reference numeral 31represents an address generation unit. The reference numeral 32represents an address buffer. The reference numeral 33 represents acoefficient averaging unit. The reference numeral 34 represents a LUT.The reference numeral 35 represents a coefficient buffer. The referencenumeral 36 represents a multiplier. The reference numeral 37 arepresents a coefficient selection unit. The reference numeral 38represents a coefficient calculation unit. The reference numerals 51 and52 represent selectors. The reference numerals 53 a, 53 b, 53 c, 53 d,53 e and 53 f represent first specific address arithmetic units. Thereference numeral 61 represents second specific address arithmeticunits. The reference numeral 62 represents a selector. The referencenumerals 71 a, 71 b, 71 c, 71 d, 71 e and 71 f represent conditiondefinition units. The reference numeral 72 represents a determinationunit. The reference numeral 73 represents a selector. The referencenumerals 74 c and 74 d represent selection condition storage units. Thereference numeral 75 d represents a characteristic value calculationunit. The reference numeral 76 f represents an update number counter.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a depicting of the superiorityand inferiority of the invention. Although the embodiment(s) of thepresent inventions have been described in detail, it should beunderstood that the various changes, substitutions, and alterationscould be made hereto without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A distortion compensating apparatus that performs predistortion by using a look-up table and optimizes the look-up table by using an adaptive algorithm, the apparatus comprising: a storage unit that stores the look-up table including distortion compensation coefficients respectively stored at addresses assigned to the look-up table; a predistortion unit that selects, using a correlation between a plurality of predetermined parameters and a plurality of the assigned addresses, addresses showing the parameters of an input signal based on the correlation from among the assigned addresses, acquires distortion compensation coefficients stored at the selected addresses, and performs the predistortion of the input signal by using the acquired distortion compensation coefficients; an error calculation unit that calculates an error signal by comparing with the input signal a feedback signal that indicates an output of a power amplifier to which a result of the predistortion is inputted; a coefficient calculation unit that calculates distortion compensation coefficients from the error signal and the acquired distortion compensation coefficients by using the adaptive algorithm; a coefficient selection unit that, for each of the selected addresses, selects distortion compensation coefficients as adequate coefficients from among the calculated distortion compensation coefficients according to the error signal; and a coefficient averaging unit that, for each of the selected addresses, calculates an average value of the adequate coefficients and replaces a distortion compensation coefficient stored at a corresponding address in the look-up table with the average value, wherein for each of the selected addresses, the coefficient selection unit calculates an evaluation value from the error signal, determines a region of the evaluation value, and selects, when the evaluation value is within the region, a distortion compensation coefficient calculated from the error signal as one of the adequate coefficients, and wherein the coefficient selection unit stores a relationship between an address in the look-up table and a threshold value defining the region and determines the threshold value based on the selected addresses by using the relationship.
 2. The distortion compensating apparatus according to claim 1, wherein the coefficient selection unit calculates absolute values of the error signal, calculates a reference value being an average value of the absolute values for each of the selected addresses, and calculates a deviation of the absolute values from the reference value as the evaluation value.
 3. The distortion compensating apparatus according to claim 2, wherein the coefficient selection unit determines a threshold value that defines the region according to the reference value.
 4. The distortion compensating apparatus according to claim 3, wherein the coefficient selection unit calculates a standard deviation of the absolute values of the error signal and determines the threshold value according to the standard deviation.
 5. The distortion compensating apparatus according to claim 3, wherein the coefficient selection unit stores a relationship between the reference value and the threshold value to determine the threshold value from the reference value according to the relationship.
 6. The distortion compensating apparatus according to claim 2, wherein the coefficient selection unit acquires a parameter indicating time since a predetermined point in time, includes the relationship between the indicated time and a threshold value that defines the region, and determines the threshold value based on the indicated time by using the relationship.
 7. The distortion compensating apparatus according to claim 1, wherein the coefficient selection unit calculates a characteristic value indicating a characteristic of the input signal, stores a relationship among the characteristic value, an address in the look-up table and a threshold value defining the region, and determines the threshold value based on the characteristic value and the selected address by using the relationship.
 8. A transmitting apparatus that performs predistortion by using a look-up table and optimizes the look-up table by using an adaptive algorithm, the apparatus comprising: a storage unit that stores the look-up table including distortion compensation coefficients respectively stored at addresses assigned to the look-up table; a predistortion unit that selects, using a correlation between a plurality of predetermined parameters and a plurality of the assigned addresses, addresses showing the parameters of an input signal based on the correlation from among the assigned addresses according to an input signal, acquires distortion compensation coefficients stored at the selected addresses, and performs the predistortion of the input signal by using the acquired distortion compensation coefficients; a power amplifier that amplifies a result of the predistortion; an error calculation unit that calculates an error signal by comparing with the input signal a feedback signal that indicates an output of a power amplifier to which a result of the predistortion is inputted; a coefficient calculation unit that calculates new distortion compensation coefficients from the error signal and the acquired distortion compensation coefficients by using the adaptive algorithm; a coefficient selection unit that, for each of the selected addresses, selects distortion compensation coefficients as adequate coefficients from among the calculated distortion compensation coefficients according to the error signal; and a coefficient averaging unit that, for each of the selected addresses, calculates an average value of the adequate coefficients and replaces a distortion compensation coefficient stored at a corresponding address in the look-up table with the average value, wherein for each of the selected addresses, the coefficient selection unit calculates an evaluation value from the error signal, determines a region of the evaluation value, and selects, when the evaluation value is within the region, a distortion compensation coefficient calculated from the error signal as one of the adequate coefficients, and wherein the coefficient selection unit stores a relationship between an address in the look-up table and a threshold value defining the region and determines the threshold value based on the selected addresses by using the relationship.
 9. A method executed by a distortion compensating apparatus for performing predistortion using a look-up table and optimizing the look-up table using an adaptive algorithm, the method comprising: selecting, using a correlation between a plurality of predetermined parameters and a plurality of the assigned addresses, addresses showing the parameters of an input signal based on the correlation from among addresses assigned to the look-up table, the look-up table being stored in a storage unit and including distortion compensation coefficients respectively stored at the assigned addresses; acquiring distortion compensation coefficients stored at the selected addresses; performing the predistortion of the input signal by using the acquired distortion compensation coefficients; calculating an error signal by comparing with the input signal a feedback signal that indicates an output of a power amplifier to which a result of the predistortion is inputted; calculating distortion compensation coefficients from the error signal and the acquired distortion compensation coefficients by using the adaptive algorithm; for each of the selected addresses, selecting distortion compensation coefficients as adequate coefficients from among the calculated distortion compensation coefficients according to the error signal; and for each of the selected addresses, calculating an average value of the adequate coefficients and replaces a distortion compensation coefficient stored at a corresponding address in the look-up table with the average value, wherein for each of the selected addresses, an evaluation value from the error signal is calculated, a region of the evaluation value is determined, and when the evaluation value is within the region, a distortion compensation coefficient calculated from the error signal as one of the adequate coefficients is selected, and wherein a relationship between an address in the look-up table and a threshold value defining the region is stored and the threshold value based on the selected addresses by using the relationship is determined. 